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Heavy Flooding and Global Warming: Is There a Connection?

March 2010 Backgrounder

Climate change increases the probability of some types of weather. Recent heavy rains and flooding in the Northeast, Midwest, and Great Plains are consistent with a warming planet, and such events are expected to become more common over time.

As average temperatures in regions across the country have gone up, more rain has fallen during the heaviest downpours. Very heavy precipitation events, defined as the heaviest one percent, now drop 67 percent more precipitation in the Northeast, 31 percent more in the Midwest and 15 percent more in the Great Plains, including the Dakotas, than they did 50 years ago.

This happens because warmer air holds more moisture. This fact is apparent when you see water vapor hanging in the air after turning off a hot shower. When warm air holding moisture meets cooler air, the moisture condenses into tiny droplets that float in the air. If the drops get bigger and become heavy enough, they fall as precipitation.

If the emissions that cause global warming continue unabated, scientists expect the amount of rainfall during the heaviest precipitation events across country to increase more than 40 percent by the end of the century. Even if we dramatically curbed emissions, these downpours will still increase, but by only a little more than 20 percent. Regardless of what action we take to cut emissions, municipalities that are vulnerable to heavy precipitation events should plan for more flooding. Any efforts to reduce emissions would make it easier for them to adapt.

Climate science contrarians often argue that it is impossible for global warming to cause both heavy precipitation and drought. They are either misinformed or purposefully confusing the issue. Drought is a measure of annual precipitation, not the intensity of precipitation events.

As a consequence of global warming, annual precipitation levels have increased in many parts of the country and decreased in others. Between 1958 and 2007, for example, the Southeast and Southwest experienced more drought even while overall precipitation across the country increased an average of five percent. These precipitation changes, along with temperature shifts, threaten agriculture and have contributed to the northerly movement of plant hardiness zones.

It is worth noting that last months blizzard in the Midwest, Mid-Atlantic, and Northeast also is consistent with global warming. That heavy precipitation event just happened to occur in winter rather than spring, bringing snow rather than rain. Global warming will likely make the winter season shorter and colder weather less common, but winter will not disappear altogether.

Global warming also is causing measurable season creep worldwide. Spring weather is arriving earlier and fall weather is arriving later than ever before. If temperatures were a bit colder in the Northeast or Midwest, those regions would have been contending with a massive snowfall instead of heavy rains.

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